DARPP-32, a dopamine- and cyclic AMP (cAMP)-regulated phosphoprotein having a molecular weight of 32 kilodaltons, is a cytosolic protein which is selectively enriched in medium-sized spiny neurons in neostriatum (Ouimet et al., 1984; Walaas and Greengard, 1984). DARPP-32 is phosphorylated by cAMP-dependent protein kinase (PKA) on a single threonine residue, thr.sup.34, resulting in its conversion into a potent inhibitor of protein phosphatase-1 (Hemmings et al., 1984). DARPP-32 can be dephosphorylated and inactivated in vitro by the calcium/calmodulin-dependent protein phosphatase, calcineurin (King et al., 1984). Dopamine has been shown to stimulate the phosphorylation of DARPP-32 in neostriatum by activation of a biochemical cascade involving stimulation of D1 receptors, activation of adenylyl cyclase, increased cAMP formation and increased activity of PKA (Walaas and Greengard, 1984). The selective enrichment of DARPP-32 in dopaminoceptive neurons and its regulation by dopamine strongly indicate that DARPP-32, by regulating protein phosphatase-1 activity, plays a key role in mediating the effects of dopamine on these cells. The control of protein phosphatase-1 activity by DARPP-32 is likely to have a significant role in the regulation of neuronal excitability. For instance, in neostriatum, dopamine-mediated effects on the function of calcium channels (Surmeier et al., 1994), voltage-dependent sodium channels (Surmeier et al., 1992; Schiffman et al., 1994) and Na.sup.+,K.sup.+ -ATPase (Aperia et al., 1991) are regulated directly or indirectly by protein phosphatase-1.
Medium-sized spiny neurons of the neostriatum and nucleus accumbens receive dopaminergic input from cell bodies in the midbrain (Anden et al., 1964; Poirier and Sourkes, 1965; Swanson, 1982). To date, five dopamine receptor subtypes have been identified which constitute two major subclasses, a D1 subfamily (D1 and D5 subtypes) and a D2 subfamily (D2, D3 and D4 subtypes) (Sibley and Monsma, 1992). D1 and D2 dopamine receptors are abundantly expressed on cell bodies and dendritic processes of medium spiny neurons (Levey et al., 1993). Messenger RNAs coding for each of the other dopamine receptor subtypes (i.e., D3, D4, and D5) have been isolated from individual neostriatal neurons (Surmeier et al., 1996), but whether these receptor proteins are expressed in medium spiny neurons and how they functionally interact with D1 and D2 receptors is still unclear.
There is considerable evidence for either synergistic or opposing interactions of D1-like and D2-like dopamine receptors at the biochemical, physiological, and behavioral level (see Jackson and Westlind-Danielsson, 1994 for review). Biochemically, D1 and D2 receptors have opposing actions on the activity of adenylyl cyclase in neostriatal neurons; whereas activation of D1 receptors increases cAMP formation by adenylyl cyclase, D2 receptors inhibit adenylyl cyclase activity (Stoof and Kebabian, 1981). Recent studies have shown that D2-like dopamine receptors via interactions with specific G-proteins, can be coupled to multiple effector systems, including calcium channels, potassium channels and phospholipase C (for review, see Huff, 1996). For example, Yan et al., (1996) have shown that D2 receptors on neostriatal neurons negatively couple to calcium channels through a G.sub.I/o class protein. In addition, activation of D2 receptors apparently decreases sodium currents in medium spiny neostriatal neurons through a membrane-delimited pathway and increases these currents through a soluble second messenger pathway (presumably involving inhibition of adenylyl cyclase) (Surmeier et al., 1992).
Schizophrenia is a clinical ailment which has an immense effect on the public health. For example, it has been estimated that as many as 50% of the homeless people living in the United States are schizophrenic. [Bachrach, In: Treating the Homeless Mentally Ill, Washington, D.C., American Psychiatric Press, 1340, Lamb et al. ed. (1992)]. In addition, approximately 2.5% of the total dollars spent for health care in the United States is spent in the treatment of schizophrenia [Rupp et al., Psychiatric Clin. North Am. 16:413-423 (1993)]. Unfortunately, at present, there exists no reliable or effective means of treating schizophrenia.
Therefore, there is a need to provide new drugs assays which can be used to develop new drugs to treat schizophrenia. Further there is a need to provide new drugs which can be used in such treatment.
The citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application.